Cutter inserts for machine and other tools may comprise a layer of polycrystalline diamond (PCD) bonded to a cemented carbide substrate. PCD is an example of a superhard material, also called superabrasive material, which has a hardness value substantially greater than that of cemented tungsten carbide.
Components comprising PCD are used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. PCD comprises a mass of substantially inter-grown diamond grains forming a skeletal mass, which defines interstices between the diamond grains. PCD material comprises at least about 80 volume % of diamond and may be made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa and temperature of at least about 1,200 degrees centigrade in the presence of a sintering aid, also referred to as a catalyst or solvent/catalyst material for diamond. Catalyst material for diamond is understood to be material that is capable of promoting direct inter-growth of diamond grains at a pressure and temperature condition at which diamond is thermodynamically more stable than graphite. Some catalyst materials for diamond may promote the conversion of diamond to graphite at ambient pressure, particularly at elevated temperatures. Examples of catalyst materials for diamond are cobalt, iron, nickel and certain alloys including any of these. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for the PCD. The interstices within PCD material may at least partly be filled with the catalyst material.
A well-known problem experienced with this type of PCD material, however, is that the residual presence of the catalyst material for diamond, in particular a metallic catalyst material for diamond, for example Co, Ni or Fe, in the interstices may have a detrimental effect on the performance of the PCD material at high temperatures. During application, the PCD material may heat up and degrade thermally, largely due to the presence of the metallic catalyst material that may catalyse graphitisation of the diamond and may also cause stresses in the PCD material due to the large difference in thermal expansion between the metallic catalyst material and the diamond microstructure.
One approach to addressing this problem is to remove, typically by leaching, the catalyst material, also referred to as a catalyst/solvent in the art, from the PCD material.
U.S. Pat. No. 3,745,623 and U.S. Pat. No. 4,636,253 teach the use of heated acid mixtures in the leaching process in which mixtures of HF, HCl, and HNO3 and HNO3 and HF, respectively, are used.
U.S. Pat. No. 4,288,248 and U.S. Pat. No. 4,224,380 describe removal of the catalyst/solvent by leaching the PCD tables in a hot medium comprising HNO3—HF (nitric acid and hydrofluoric acid), alone or in combination with a second hot medium comprising HCl—HNO3 (hydrochloric acid and nitric acid).
US 2007/0169419 describes a method of leaching a portion or all of the catalyst/solvent from a PCD table by shielding the portion of the PCD table not to be leached and immersing the shielded PCD table in corrosive solution to dissolve the catalyst/solvent in water and aqua regia. The leaching process is accelerated by the use of sonic energy, which agitates the interface between the PCD table and the corrosive solution to accelerate the dissolution rate of the catalyst/solvent.
U.S. Pat. No. 4,572,722 discloses a leaching process that is accelerated by forming a hole in the PCD table by laser cutting or spark emission prior to or during the leaching process. The PCD table is then leached by using conventional acid leaching techniques, electrolytic leaching and liquid zinc extraction.
An alternative approach to addressing the problem is to use a non-metallic catalyst material for diamond that produces a more thermally stable PCD material.
JP2795738 (B2) describes sintering a mixture of diamond powder and metal carbonates at pressures of 6-12 GPa and temperatures of 1700-2500° C. to give sintered polycrystalline material consisting of 0.1-15 vol % non-metallic binder in a sintered diamond layer.
JP4114966 describes the use of carbon powder added as a sintering aid to diamond powder and an alkali earth carbonate, in order to improve the sinterability of the non-metallic system.
JP2003226578 also addresses the problem of poor sinterability, which describes the use of oxalic acid dihydrate as a sintering aid in a carbonate-based non-metallic solvent/catalyst system.
JP2002187775 describes the addition of other organic compounds to achieve a sintered carbonate-based non-metallic PCD, and similarly the addition of metal carbides is described in JP6009271.